Optical gratings are used for light coupling and delivery in a variety of optical systems. For example, in energy assisted magnetic recording (EAMR) electromagnetic radiation (light) is provided from a laser to a conventional grating. Typically, the light provided from the laser is in the optical range of the spectrum. The conventional grating is configured for a particular wavelength in the spectrum. Typically this means that the conventional grating actually functions in a range of wavelengths around the particular wavelength. The conventional grating couples light of the particular wavelength from the laser to a waveguide. The light from the waveguide is typically provided to a near-field transducer (NFT) and used to heat a spot on a magnetic recording media. Data is magnetically written to the spot while the spot is heated.
FIG. 1 depicts such a conventional grating 10 formed on a substrate 12. The conventional grating 10 may be used in magnetic recording applications. The conventional grating 10 includes a conventional optical core 14. Further, top and bottom cladding may also be included. However, such layers are not depicted for simplicity. The conventional core includes ridges 16 interspersed with troughs 18 and spaced apart a pitch, d. The conventional grating 10 is configured for use with light 20 having a wavelength, λ. The light 20 is a beam represented by rays 22, 24, and 26. The light 20 is generated by a laser and travels to the conventional grating 10. Central ray 22 corresponds to the general direction in which the light 20 travels. However, the light 20 from the laser is also characterized by a divergence represented by angle φ. Thus, the rays 24 and 26 diverge from the central ray 22 by the angle φ. The conventional grating 10 couples the light 20 into a waveguide (not shown), which redirect the light 20 for use in writing data to a media (not shown).
FIG. 2 depicts a conventional method 50 for fabricating a conventional grating such as the conventional grating 10. The core materials, such as Ta2O5 are deposited, via step 52. A photoresist mask is provided on the core material, via step 54. The photoresist mask has a series of lines interleaved with apertures. The core material is etched, via step 56. Thus, the pattern of the photoresist mask is transferred to the core material. The conventional core 14 may thus be fabricated.
Although the conventional grating 10 and method 50 function, improvements are desired. The coupling efficiency of a grating is a measure of the losses in optical energy between light input to the grating and light output by the grating. A higher coupling efficiency translates to lower losses in a grating. Thus, a higher coupling efficiency is desired. In order to achieve high coupling efficiency in a grating, the geometry of the grating, such as the pitch, depth, and shape of ridges and troughs in a grating are closely controlled. In the conventional grating 10, the pitch, d is generally set to optimize coupling efficiency for the wavelength, λ, and the principal angle of incidence γ1. However, because of the divergence, the rays 22, 24, and 26 are incident upon the grating 10 at different angles. For example, central ray 22 has an angle of incidence with the core 14 of γ1. The ray 24 has an angle of incidence with the core 14 of γ2. The ray 26 has an angle of incidence with the core 14 of γ3. As shown in FIG. 1, γ1, γ2, and γ3 may differ. Similarly, portions of the light 20 between the rays 24 and 26 have varying angles of incidence. As a result, different portions of the light 20 are coupled into the conventional grating 10 with varying efficiency. The optical efficiency of the conventional grating 10 may, therefore, degrade.